How to make a node group to calculate fresnel via F0 values
Update 1:



The built in Fresnel node contains the equation for dielectrics, not metals.
However in most cases the the curve is almost identical.

It is possible to implement metallic Fresnel using nodes, but it's complicated and not worth it in my opinion.

Each RGB component of the specular color is treated as its own $F_0$.
(make sure all RGB values are greater than zero, otherwise it will not converge to white at the edges)

I derived an equation that relates $F_0$ to $n$ (index of refraction) from this equation (Derived from Snell's law):

$$
F_0 = \left( \frac{n_1 - n_2}{n_1 + n_2} \right)^2
$$

Where $n_1$ is the index of refraction of the material the light is coming from and $n_2$ is the index of refraction of the material the light is hitting. In most cases air, which has an index of refraction of approximately one, by substitution in the equation:

$$
F_0 = \left( \frac{1 - n_2}{1 + n_2} \right)^2
$$ 

And since $n_2$ is the index of refraction of the material the light is hitting:

$$
F_0 = \left( \frac{1 - n}{1 + n} \right)^2
$$

Using algebra we can rearrange the equation to find $n$:

$$
n = \frac{1+\sqrt{f_0}}{1 - \sqrt{f_0}}
$$

$n$ is not an independent variable. This means $F_0$ and $n$ can not be controlled separately. Changing them independently would cause an incorrect curve. (Yes, that means the Fresnel in the layer weight node is incorrect)

Unfortunately Blenders implementation of the Fresnel equation does not take into account roughness. This would be an easy fix if we knew the direction to the light source, but we don't have a node with that info. So we just have to approximate it as best we can. This is what Cynicat's roughness hack does.

I have exposed the index of refraction in the Fresnel node group because it's useful for things like refraction.

If you want an edge tint like some metals have you can approximate it by mixing two colors based on the facing value in the layer weight node, then passing that color into the specular slot. 

If you want some specular values for materials there is a small list here "Background: Physics and Math of
Shading" on page 17. [make sure you use the linear values not the SRGB ones]

Update 2:

Perhaps you could try Schlick's approximation. It's what most game engines use. It doesn't have that downward curve with high $F_0$ values.